Present invention relates generally to signal propagation system's and more specifically to an improved fast signal low impedance propagation system.
The signal propagation, for example, in carries between stages or increments between stages of logic in counters, adders, etc, generally require that the propagation be as fast as possible while not adding substantial impedance to the line when a plurality of stages are included. For fast transmission, the prior art uses a transmission gate as illustrated in FIG. 1. The logic may be for example, an MOS device having its gate controlled by the propagation signal to immediately transmit or propagate the input signal which is a carry-in to the output terminal as a carry-out. A generate logic is provided in parallel to the transmission line to provide a carry-out as a function of the generate logic. Typical examples are U.S. Pat. No. 3,843,876 to Fette et. al. and U.S. Pat. No. 4,152,775 to Schwartz.
Another method of the prior art of propagation of information through stages of logic includes inverting logic, for example NAND or NOR gates at each stage. As illustrated in FIG. 2, the input signal is inverted and provided to inverting parallel propagation logic which provides an output signal to the signal out terminal. The inverting parallel propagation logic is similar to the generation logic in that it ties the output terminal to a specific value as a function of the carry-in and the propagation signal P. A typical example is shown in U.S. Pat. No. 4,054,788 to Maitland et. al. The use of inverting logic wherein the carry signal is inverted every stage is illustrated in U.S. Pat. No. 4,052,604 also to Maitland et. al.
Although the transmission gates provides a fast transition through each stage, the propagation through a long chain of logic stages tends to become slow because of the loss of signal level or increase of series impedance in the later stages. The inverting logic avoids the problems of impedance increase by recharging the logic levels at each stage but suffers from turnon delays of the inverting logic of each stage.
Thus there exists the need for a propagation logic which is capable of fast transmission without the build-up of impedance at the later stages of logic.